Astronomers witness slow buildup and smoldering aftermath to explosive stellar climax
K. Ulaczyk/Warsaw University Observatory

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Astronomers witness slow buildup and smoldering aftermath to explosive stellar climax

The classical novae of binary star systems are thought to be cyclical explosions that repeat every 10,000 to a million years. But information on how the binaries behave between eruptions—particularly just before an outburst—is understandably scarce. Now, a team of astronomers has collected observations in the 6 years before and 7 years after a recent blast, dubbed Nova Centauri 2009, because they were lucky enough to be looking in the right part of the sky. Novae occur when a burned-out remnant of a star known as a white dwarf cannibalizes its stellar companion, pulling in some of the other star’s hydrogen with its strong gravity (seen in artist’s illustration, above). The hydrogen coats the surface of the white dwarf—usually made of carbon and oxygen—until it is thick and hot enough to spark a runaway fusion reaction: an explosion that blows off the coat and causes the white dwarf to become 10,000 times brighter. But what happens next? Theorists think that the system does not just go back to square one. Instead, they say, the explosion heats up the companion star for several centuries, causing an increased rate of hydrogen transfer to the white dwarf. The rate should then slow to a trickle, putting the binary pair into “hibernation” until the white dwarf accumulates a critical mass of hydrogen. That scenario has now gained support from a team using the Optical Gravitational Lensing Experiment telescope in Chile. They report online in Nature today that in the years before Nova Centauri exploded, it showed short, irregular increases in brightness typical of a low-hydrogen transfer rate. In the years after, the pair was two orders of magnitude brighter and rock-steady, indicating a much higher hydrogen flow. Now, the nova is starting to fade to its hibernating state, where it will sit until its next charismatic explosion.